In semiconductor technology, interconnects are formed on a substrate to electrically connect various active components of a semiconductor device. The interconnects are formed as conductive lines, which extend substantially parallel to a top surface of the substrate, and conductive vias, which electrically connect conductive lines on different metal layers. A metal layer is a group of conductive lines having a same distance from the top surface of the substrate. A group of conductive lines closest to the substrate is often called metal layer zero (M0).
Conductive lines are formed by creating openings in a dielectric material and filling those openings with a conductive material. A planarization process is used to remove excess conductive material following filling of the openings. As critical dimensions of semiconductor devices continue to shrink, filling the openings becomes more challenging.